A Split Hopkinson pressure bar is a mechanical test instrument that is used to characterise the dynamic response of materials when subjected to high strain rates.
FIG. 1 shows a schematic arrangement of a conventional Split Hopkinson pressure bar. A striker bar 10 is projected towards, and impacts against, a first end of the incident bar 12. Typical impact velocities range from 10 m/s to 20 m/s. This impact creates a stress wave that propagates along the incident bar towards the test specimen 14 that is positioned between the opposite second end of the incident bar 12 and a first end of the transmission bar 16.
When the stress wave reaches the test specimen 14 it splits into two smaller stress waves. The first of these smaller stress waves is transmitted through the test specimen 14 and into the transmission bar 16. The second of these smaller stress waves is reflected away from the test specimen 14 and back along the incident bar 12.
A stop bar 18 is provided at the second end of the transmission bar 16 to absorb the impact of the transmission bar 16.
Strain gauges are used at the incident bar 12 and the transmission bar 16 to record the stress waves.
A known problem with the Hopkinson pressure bar apparatus is that at impacting velocities above 20 m/s, the stresses generated in the incident and transmission bars may damage the bars themselves or the associated strain gauges.
One solution to this problem is to remove the incident bar and directly impact the test specimen with the striker bar. However, this arrangement, known as the Direct Impact Hopkinson bar, requires the use of high speed photography in the absence of the incident bar's strain gauge data. This increases the cost and complexity of such high strain rate testing.
Furthermore, once the test specimen is fully compacted, the impact loading may damage the transmission bar.